Energetic diode-pumped lasers use laser diodes in various geometries, mostly arranged around the laser rod, performing side pumping perpendicular to the rod axis. The light emitted by the laser diodes enters perpendicular to the rod axis. The pump light is absorbed by the atoms in the laser rod, exciting the atoms, thus establishing an optical gain in the laser rod. The side pumping geometry allows a large excited cross-section of the laser rod as well as long rod pumping, facilitating large pumped volume and large energy storage and high-energy extraction as required.
The side-pumped, diode-pumped solid-state laser (side-pumped DPSSL) field can be divided into sub-fields based on how the otherwise highly divergent, up to 40°, diode radiation is coupled into the laser rod. Some of these sub-fields include: (a) using optics such as a cylindrical lens or elliptical mirror, (b) using an optical waveguide such as a reflective cavity or fiber; and (c) closely coupling the diode(s) to the rod.
The side-pumped, diode-pumped solid-state laser (side-pumped DPSSL) field can be further divided into sub-fields based on how the heat, resulting from the method used to remove the part of the electric energy introduced into the cavity that is not transferred to laser light is removed. Some of these sub-fields include: (a) using liquid, circulating in and out of the cavity, where the excessive heat is convected away, (b) using gas, circulating in and out of the cavity, where the excessive heat is convected away, (c) using a solid-state structure, where the excessive heat is conducted away through the solid structure. Heat removal in most prior art arrangements and structures was performed using compressed gas of liquid coolants. Gas or liquid coolants limit the reliability of the laser system, since frequent preventive maintenance activity is required to address leaks of the coolant or degradation of its characteristics.
Japanese patent publication no. JP 5-259540 discloses a side-pumped DPSSL wherein the rod is disposed within a diffuse reflector or condenser. A diode array emits radiation that enters the condenser for absorption by the rod via a narrow slit which guides the diode radiation toward the rod. A gel or liquid such as water surrounds the rod filling spacing between the rod and surrounding tube. Some light is absorbed in the liquid, scattered and absorbed by multiple reflections.
U.S. Pat. Nos. 5,521,936, 5,033,058 and 6,026,109 disclose water-cooled solid-state lasers using closely coupled side-pumping diode arrays. The pumping laser diodes are disposed close to the rod in order that the rod remains in the path of the substantial portion of the divergent radiation, as it is not contemplated that rays missing the rod on the first pass will be subsequently redirected towards the rod to be absorbed on a second or later pass, the efficiency is reduced. U.S. Pat. No. 5,870,421 patent differs in that it discloses the use of side-pumping optical fibers, which add substantial manufacturing cost.
In addition, the alternative configurations described in U.S. Pat. Nos. 5,521,936, 5,033,058 and 6,026,109 include a rod which is cooled with a water jacket enclosed a flow tube, with the diodes and reflector disposed outside the flow tube, similarly to JP 5-259540. Here, a disadvantage is that the wall thickness of the flow tube adds significant distance between the rod and reflector. In configurations using a diffuse reflector, this leads to increased losses of the pump light, resulting in reduced efficiency.
U.S. Pat. No. 6,608,852 describes a liquid-cooled side-pumped laser including an elongated diffuse reflector housing having an elongated cavity defined by a diffusely reflective cavity wall, with a solid-state rod disposed within the cavity and surrounded by a cooling fluid flowing along the rod for cooling the rod. This laser has the advantage of uniform pumping, but the liquid cooling is problematic in various environmental conditions (like freezing).
In the close-coupled arrangement described in U.S. Pat. No. 5,774,488, the rod is enclosed in a heat-conducting specular reflector, and the pump radiation is introduced through a narrow slit in the reflector. This arrangement produces specular reflection, that causes non-uniform pumping of the rod cross section, and is complex to manufactures. Additionally, the specular reflector enhances the ASE (Amplified Spontaneous Emission) from the rod by providing parasitic laser paths. Additionally, any difference in thermal expansion of the rod and reflector may cause mechanical stress. Also, pump radiation from the diode must pass through a long narrow slit (channel) in the metal reflector, thus suffering multiple reflections and therefore extra losses.
Another problem is the type of reflector that can be used to pump light, while enabling uniform pumping of the rod cross section. Specular reflectors are not able to produce the same level of uniformity of the pump radiation as diffuse reflectors. For example, Hanson, et al., citation below, discloses a three-bar diode array placed a small distance away from a large opening to a solid-state laser cavity. Ajer et al., citation below, discloses a closely coupled side-pumping diode array which pumps the rod through a slit-like opening. The cavities disclosed by Hanson, et al. and Ajer, et al. include highly reflective inner surfaces, and the intensity distributions of the pumping diode radiation within the rods lack homogeneity.
Generally, pumping with a diode array from one direction can lead to a cylindrical intensity distribution (as shown, for example, in the paper by Hanson, et al.). This gives rise to a cylindrical thermal lens in the rod, which, in turn, results in an astigmatic output beam of the laser. To improve circularity, some of the mentioned references describe alternative arrangements which use pumping radiation from several (two or more) directions. The problem with this approach, however, is that laser diodes tend to age differently, which destroys the intensity balance over the lifetime of diodes.
In U.S. Pat. Nos. 5,317,585 and 5,781,580, a transparent heat conductor is used, since the heat conductor of these designs has to be optically transparent to allow the diode light to enter the laser rod and at the same time conductively cool the rod. High optical transparency and high thermal conductivity properties are not readily found in one material (except in diamond which is extremely expensive and cannot be machined to the needed shapes), and thus the solution is not optimized for any of the parameters.
Other references are:    Walter Koechner, “Solid-state Laser Engineering”, pp. 127-140, 709 (Springer series in optical sciences, v. 1, Springer-Verlag, Berlin, Heidelberg, N.Y., 1996).    Frank Hanson and Delmar Haddock, “Laser diode side pumping of neodymium laser rods”, Applied Optics, vol. 27, no. 1, 1988, pp. 80-83.    H. Ajer, et al., “Efficient diode-laser side-pumped TEM00-mode Nd:YAG laser”, Optics Letters, vol. 17, no. 24, 1992, pp. 1785-1787.    Jeffrey J. Kasinski, et al., “One Joule Output From a Diode Array Pumped Nd:YAG Laser with Side-pumped Rod Geometry”, J. of Quantum Electronics, Vol. 28, No. 4 (April 1992). D. Golla, et al., “300-W cw Diode Laser Side-pumped Nd:YAG Rod Laser”, Optics Letters, Vol. 20, No. 10 (May 15, 1995).    Japanese Patent No. JP 5-259540.    U.S. Pat. Nos. 5,774,488, 5,521,936, 5,033,058, 6,026,109, 5,870,421, 5,117,436, 5,572,541, 5,140,607, 4,945,544, 4,969,155, 5,875,206, 5,590,147, 3,683,296, 3,684,980, 3,821,663, 5,084,886, 5,661,738, 5,867,324, 5,963,363, 5,978,407, 5,661,738, 4,794,615, 5,623,510, 5,623,510, 3,222,615, 3,140,451, 3,663,893, 4,756,002, 4,755,002, 4,794,615, 4,872,177, 5,050,173, 5,317,585, 5,349,600, 5,455,838, 5,488,626, 5,521,932, 5,590,147, 5,627,848, 5,627,850, 5,638,388, 5,651,020, 5,838,712, 5,875,206, 5,677,920, 5,781,580, 5,905,745, 5,909,306, 5,930,030, 5,987,049, 5,995,523, 6,009,114, and 6,002,695.    German Patent No. DE 689 15 421 T2.    Canadian Patent No. 1,303,198.    French Patents Nos. 1,379,289 and 2,592,530.    Fujikawa, et al., “High-Power High-Efficient Diode-Side-Pumped Nd:YAG Laser”, Trends in Optics and Photonics, TOPS Volume X, Advanced Solid-state Lasers, Pollock and Bosenberg, eds., (Topical Meeting, Orlando, Fla., Jan. 27-29, 1997).    R. V. Pole, IBM Technical Disclosure Bulletin, “Active Optical Imaging System”, Vol. 7, No. 12 (May 1965).    Devlin, et al., “Composite Rod Optical Masers”, Applied Optics, Vol. 1, No. 1 (January 1962).    Goldberg et al., “V-groove side-pumped 1.5 um fibre amplifier,” Electronics Letters, Vol. 33, No. 25, Dec. 4, 1997).    Welford, et al., “Efficient TEM00-mode operation of a laser diode side-pumped Nd:YAG laser, Optics Letters, Vol. 16, No. 23 (Dec. 1, 1991).    Welford, et al., “Observation of Enhanced Thermal Lensing Due to Near-Gaussian Pump Energy Deposition in a Laser Diode Side-Pumped Nd:YAG Laser,” IEEE Journal of Quantum Electronics, Vol. 28, No. 4 (Apr. 4, 1992).    Walker, et al., “Efficient continuous-wave TEM00 operation of a transversely diode-pumped Nd:YAG laser,” Optics Letters, Vol. 19, No. 14 (Jul. 15, 1994).    Comaskey et al., “24-W average power at 0.537 um from an externally frequency-doubled Q-switched diode-pumped ND:YOS laser oscillator,” Applied Optics, Vol. 33, No. 27 (Sep. 20, 1994).
Novel solutions allowing efficient side pumping, uniform pump distribution across the rod, pumping with a single source and one side and good conductive cooling are needed and are presented in this invention. The optimization of the mentioned parameters results in smaller volumes and weight as well.